1. Field of the Invention
This invention relates to a switching power supply circuit, more particularly to a switching power supply circuit in which a power factor of the power supply and a voltage regulation are improved.
2. Description of Related Art
Increasing the switching frequency in a switching power supply may allow a transformer or other devices to be reduced in size enabling the switching power supply to be used as a power supply for various electronic equipment by acting as an AC-DC converter of high electrical power.
In general, when a commercial power supply is rectified, an electrical current flowing in a smoothing circuit shows a distorted waveform, which may produce a problem in that a power factor indicating an utilization efficiency of the power supply is deteriorated.
In addition, it is required to provide a countermeasure to prevent an occurrence of a higher harmonic generated when the current shows a distorted waveform.
In order to improve the power factor of the power supply, the simplest method is to use a choke input type rectifying circuit, for example, and this method is preferable in view of applying a countermeasure against an electromagnetic noise (EMI). However, this system requires an inductor having a high impedance as a choke coil, resulting in that a size reduction of electronic equipment is prohibited by this requirement and at the same time also causing its cost to be increased.
In particular, in the case of a power supply of a 100-V system or a 200-V system, it is necessary to prepare different kinds of choke coils for each of the systems and, for a power supply for a TV set or the like, a costly shield is necessary for preventing oscillation of a TV screen caused by a leaked magnetic flux.
In view of such a situation as described above, a smoothing circuit of a capacitorless system in which an output of a rectifying circuit is directly connected or disconnected so as to drive the switching power supply, or an active filter or partial rectifying system for connecting or disconnecting the output of the rectifying circuit at a high frequency is employed.
The capacitorless system is constructed such that a smoothing capacitor for the power supply for use in driving the switching power supply is eliminated, wherein it shows a high effect of improving the power factor and a ripple voltage of twice the frequency of a commercial power supply is overlapped on the output of a secondary side. However, it shows a poor regulation and at the same time it does not tolerate an instant disconnection of an input voltage and so this system can not be used as a power supply device of high capacity.
The active filter system is operated such that both an input voltage and an input current are detected and a switching control is carried out in such a way that a waveform of the input current may approach a waveform of the input voltage, wherein its power factor may be set approximately to 1. This, requires two units of converters, resulting in a circuit that is complex in its configuration and an utilization efficiency of the power supply that is deteriorated. In addition, switching noise is increased and a countermeasure against the increased noise (EMI) may increase its cost.
In addition, although the partial smoothing circuit is constructed such that a choke coil arranged in a circuit for charging the smoothing capacitor is switched to expand a continuity angle of a rectifying element, there remain the problems of applying a countermeasure against noise, a reduction in efficiency, and an increased ripple voltage.
In addition, there is a certain difficulty in providing a concurrent improvement of both power factor and efficiency, and this difficulty may not be acknowledged in view of the aforesaid countermeasure against EMI. In view of this fact, a magnet switch system (hereinafter called as an MS system) has been proposed in which an interrupted voltage of the switching power supply is utilized to reduce a charging voltage of the smoothing capacitor and a continuity angle of a rectifying element is expanded to improve the power factor.
FIG. 6A shows one example of the aforesaid switching power supply circuit of the MS system, wherein a power used in be supplied to the switching power supply circuit is constructed such that a commercial power supply AC is full-wave rectified through a bridge rectifying diode D1 and at the same time this rectified voltage is supplied to a smoothing capacitor C1 through a choke coil CH and a third winding N3 of an isolating transformer CT.
Q1 denotes a switching element (MOSFET) for interrupting a voltage charged in the smoothing capacitor C1 through a primary winding of the isolating transformer CT. An alternating voltage induced in a secondary winding of the isolating transformer is rectified through rectifying diodes D4, D5, and smoothed by a coil L and a capacitor C3, resulting in a DC output voltage E0.
Then, this output voltage E0 controls a control circuit for generating a driving pulse of the switching element through a photocoupler, and performs a PWM modulation for changing a pulse width of the driving pulse, thereby a characteristic of constant voltage can be attained.
As shown in FIG. 6B, this switching power supply circuit is operated such that an electrical current I.sub.AC charged to the smoothing capacitor C1 flows with a waveform resembling a voltage waveform V.sub.AC Of the commercial power supply to be supplied. That is, since the electrical current charged to the smoothing capacitor C1 is interrupted by a switching voltage of the switching power supply circuit generated in the third winding N3, its mean current waveform I.sub.AC flows even in the case when an amplitude of V.sub.AC is low as shown in FIG. 6B and therefore the current waveform I.sub.AC approximates that of V.sub.AC.
As a result, the power factor of the switching power supply acting as an AC load is improved.
However, since this power supply system of MS type is constructed such that the third winding N3 is supplied to the aforesaid isolating transformer CT, the current flowing in the primary winding N1 is varied at a period of twice of that of the commercial power supply, and its peak current becomes approximately twice of that of the conventional type, resulting in noise generated by the transformer being increased and a heat generated at the windings and the core also being increased.
In addition, since the voltage is also varied in a similar period, a ripple voltage having this period is increased at the output voltage E0.
In addition, since each of the diodes constituting the bridge rectifying diode D1 is interrupted at the switching frequency, it is necessary to provide an expensive rectifying element in which each of the diodes can perform a high speed switching of high current.